1. Field of Invention
The present invention relates to Micro-Electro-Mechanical Systems (MEMS) device. More particularly, the present invention relates to the structure and fabrication method for MEMS device with improved sensing capability.
2. Description of Related Art
MEMS device, such as MEMS microphone, has been popular gradually due to the excellent features comparing to the conventional ECM microphone. The features of MEMS microphone includes: 1. thin and small size; 2. SMD (surface mountable device) indicating easy assembly with sold flow; 3. high stability and environmental resistance. However, in comparison with the IC package, the requirements of microphone package include receiving the sound pressure from acoustic signal, inducing mechanical motion and transferring to electrical signal. Therefore it needs an acoustic path to receive the sound pressure, a transducer to response the sound pressure, a sufficient back volume for transducer to reduce the damping coefficient and a good shielding to protect it from EMI. Currently, the most popular package is that the transducer is mounted on PCB and electrically couple to such PCB, and the conductive housing with an aperture is attached to the PCB enclosing the transducer.
FIG. 1 is a cross-sectional view, schematically illustrating a structure of MEMS device. In FIG. 1, the substrate 100, such as silicon substrate, has an indent space define by the sidewall 108 corresponding to a diaphragm region. The substrate 100 has multiple through holes 110 within the indent space define by the sidewall 108, which is defined by a side wall. A dielectric structural layer 102 is formed on the substrate 100. A MEMS diaphragm 106 is held by the dielectric structural layer 102 at the periphery. As a result, a chamber between the MEMS diaphragm 106 and the substrate 100 is formed, in which a gap with a distance d is formed. The chamber 112 is connected to the indent space define by the sidewall 108 by the through holes 110, so that the vibrating air induced by the MEMS diaphragm for sensing voice is possible. A capacitance formed between the MEMS diaphragm 106 and the substrate 100 is also changed. The variation of the capacitance can be converted into electric signal. Then, the MEMS device in FIG. 1 can serve as a MEMS microphone.
FIG. 2 is a schematic drawing for an electric circuit of MEMS microphone based on the MEMS device. In FIG. 2, the circuit includes a MEMS sensing capacitor Cmic, a circuit capacitor with capacitance Ccir, and an amplifier Amp, and a resistor with resistance R. The MEMS sensing capacitor with the capacitance Cmic can sense an audio signal. According to the circuit, the audio signal is converted as the voltage signal Vout as the output.
As can be seen in FIG. 2, the signal Vout is proportional to ΔCmic. If the capacitance Cmic is larger, then the output signal Vout can be amplified a large level. The sensitivity can then be improved. Since the capacitance is inversely proportional to the distance d, the capacitance can be increased by reducing the distance. However, the distance cannot be reduced without restriction. If the distance d of gap of the MEMS capacitor is too small, the structure may be not easily fabricated.
Conventionally, to improve the sensitivity of microphone, the most effective method is to decrease the air gap (d). However, the decreasing of air gap will result in the decreasing of pull-in voltage of diaphragm, indicating lower VPP is used. The decreasing of VPP will decrease the sensitivity, because the pull-in voltage is proportional to strength of electrical field (E=VPP/d). If d is decreasing, E will be increasing.
In addition, a surface current leakage should be considered. FIG. 3A is a cross-sectional view, schematically illustrating a structure of MEMS device with current leakage. In FIG. 3A, when the MEMS diaphragm is at the voltage VPP and the substrate is at the ground voltage, a current leakage may occur on the surface of the dielectric sidewall as indicated by arrow leak.
FIG. 3B is a schematic drawing for an electric circuit of MEMS microphone based on the MEMS device with current leakage. When the current leakage occurs, a parasitic resistor with leakage resistance Rleak is coupled with the capacitor Cmic in parallel. The sensitivity is degraded and the output noise may also increase.
Since the capacitance is related to sensitivity and is related to a distance between the MEMS diaphragm and the substrate. How to have high capacitance for the MEMS device without causing current leakage is still an issue to improve.